1. Field of the Invention
The present invention relates to the field of digital modulators/demodulators (modems) and more specifically to the operation of detecting the presence of a transmission within the modem receiver.
2. Discussion of the Related Art
In a modem using a signal processor, the data to be transmitted is digitally coded from a number of possible symbols, then transmitted as samples of portions of a carrier modulated in phase and amplitude (QAM) or in phase difference (DPH), then converted into analog signals.
On the receive side, the end of a transmission, or the interruption of a transmission, should be likely to be detected. The detection is performed on the basis of the energy of the receive signal. A transmission detector indicates through an output with two states, respectively ON and OFF, the presence or the absence of a transmission, which is characterized by the presence or the absence of a carrier.
Each communication standard determines a minimum threshold for the signal level, below which the transmission is to be regarded as interrupted. The threshold is, for example, -48 dBm for standard V27. The standard also fixes a maximum delay for identifying the transmission loss, for example from 5 to 15 ms for standard V27, and a maximum threshold above which the modem is to assume that a (ON or OFF) indicating whether a transmission is occurring or not. State machine 4 interprets the result of the comparisons of a current cycle by using, when the energy level is between the detection DETH and loss LOSSTH thresholds, the value of the previous OUT detection signal during the previous cycle in order to determine, through the evolution of the level, whether there is loss or detection of a transmission.
A problem to solve in conventional detectors is that the necessary presence of averaging block 3 obliges to compromise between respecting the switching time of the detector and the accuracy of the detector.
Indeed, a recursive first order low-pass digital filter is constituted, as shown in FIG. 1, by an integrator 5 associated with two multipliers 6 and 7 and to a shift register 8. The signal to be filtered is multiplied by a coefficient k, which is lower than 1, before being sent on a first input of integrator 5. A second input of integrator 5 receives its own output signal, multiplied with a coefficient 1-k, and delayed by register 8. The value of coefficient k determines not only the time constant of the filter, but also the ripple of the output signal of the filter.
Indeed, the lower the coefficient k, the lower the contribution of the current input value, and the longer it will take for the output of averaging block 3 to reach the input value. Thus, k must be selected with a big enough value to comply with the constraint on the detector switching time (from 5 to 15 ms for the V27 standard).
In contrast, the greater the value of coefficient k, the more the output signal of averaging block 3 will comprise strong ripple as an effect of the contribution of the current input value when the receive signal is modulated. Hence, k must be selected with a small enough value transmission occurs. The maximum threshold is for example -43 dBm for standard V27. Moreover, the switching of the detector must occur with a minimum hysteresis of 2 dB. In other words, the switching of the detector from its OFF position to its ON position must happen at a level which is at least 2 dB higher than the switching level of the detector from its ON position to its OFF position. The use of two thresholds is linked to the ripple exhibited by the signal indicating the energy of the receive signal, because of the carrier modulation.
All these constraints lead to the requirement that the detector of the presence of a transmission be able to both switch rapidly and be sufficiently accurate.
FIG. 1 schematically shows a conventional detector of the presence of a transmission in a modem. The detector includes an energy calculation circuit 1 including a circuit 2 for determining the absolute value (supplying the absolute value of the receive signal Rx), having its output sent on an averaging block 3 constituted by a first order recursive low-pass filter with a zero cut-off frequency. The signal Rx received by energy calculation circuit 1 corresponds to the signal received by the modem after digitization. Averaging block 3 is used to smooth the ripple at the output of circuit 2, as well as to avoid an ill-timed switching of the detector. Two comparators C1 and C2 of the detector receive, each one on a positive input, the output of averaging block 3 which constitutes the output of energy calculation circuit 1 for the receive signal Rx. The negative input of each comparator C1 and C2 receives a threshold value, respectively for detection (DETH) and loss (LOSSTH), with which the energy of the receive signal is to be compared. The output of the two comparators C1 and C2 is sent to a state machine 4 having the role of issuing an OUT detection signal with two states to avoid an output signal ripple of averaging block 3 would cause an ill-timed switching of comparators C1 or C2. The values of thresholds DETH and LOSSTH of comparators C1 and C2 are generally chosen inside the range separating the thresholds fixed by the standard (-43 dBm and -48 dBm for standard V27) with a distance from each other of a value greater than that of the required hysteresis (that is, &gt;2 dB for standard V27). For example, for the V27 standard, the detection threshold DETH is fixed at -44 dBm and the loss threshold LOSSTH is fixed at -47 dBm.
Moreover, the higher the rate of the modulated signal, the more the peak factor of the receive signal increases and the more the ripple amplitude of the output signal of averaging block 3 increases. Accordingly, the more the rate increases, the lower coefficient k must be. Complying with this last requirement, in presence of a modulated signal with a rate of 2400 bits per second, would lead for the V27 standard to adopt a value of 0.002 for k. This would bring about a time constant, or response time, of 240 ms for the filter, instead of the required maximum 15 ms. This phenomenon is even more important in the case of a QAM modulation (for example, according to the V17 standard) than in the case of a DPH modulation (for example, according to the V27 standard).
The compromise which must then be made leads for the conventional detectors to only meet the specified requirements with a pure, i.e. non modulated, carrier. The time constant of the filter is set in order to comply with the constraint on the switching time, at the expense of the filter accuracy in presence of a modulated signal.